Work on the mechanism of cholesterol transport from plasma lipoproteins to human erythrocytes was brought to a conclusion with the finding that transport rates were found to be independent of ionic strength of the plasma. This is in accord with our earlier conclusion that the rate determining step is the diffusion of cholesterol in the erythrocyte membrane. Further, direct determination of lipoprotein binding sites on erythrocytes by other workers were in agreement with those predicted on the basis of the membrane diffusion limit. A study was begun of the effects of species type and modifications of mammalian erythrocyte membranes on the rates of transmigration of phospholipids between the inner and outer halves of the membrane phospholipid bilayer. This work should result in molecular level information on the structure of cell membranes.